[This question](http://cogsci.stackexchange.com/questions/84/is-it-possible-to-measure-the-working-memory-of-a-non-human-animal) talked about how a "delayed match-to-sample task" can be used to measure an animal's [working memory](http://en.wikipedia.org/wiki/Working_memory). What is a delayed match-to-sample task and how can it be used to measure working memory?